Printk Index

There are many ways how to monitor the state of the system. One important source of information is the system log. It provides a lot of information, including more or less important warnings and error messages.

There are monitoring tools that filter and take action based on messages logged.

The kernel messages are evolving together with the code. As a result, particular kernel messages are not KABI and never will be!

It is a huge challenge for maintaining the system log monitors. It requires knowing what messages were updated in a particular kernel version and why. Finding these changes in the sources would require non-trivial parsers. Also it would require matching the sources with the binary kernel which is not always trivial. Various changes might be backported. Various kernel versions might be used on different monitored systems.

This is where the printk index feature might become useful. It provides a dump of printk formats used all over the source code used for the kernel and modules on the running system. It is accessible at runtime via debugfs.

The printk index helps to find changes in the message formats. Also it helps to track the strings back to the kernel sources and the related commit.

User Interface

The index of printk formats are split in into separate files. The files are named according to the binaries where the printk formats are built-in. There is always “vmlinux” and optionally also modules, for example:

/sys/kernel/debug/printk/index/vmlinux
/sys/kernel/debug/printk/index/ext4
/sys/kernel/debug/printk/index/scsi_mod

Note that only loaded modules are shown. Also printk formats from a module might appear in “vmlinux” when the module is built-in.

The content is inspired by the dynamic debug interface and looks like:

$> head -1 /sys/kernel/debug/printk/index/vmlinux; shuf -n 5 vmlinux
# <level[,flags]> filename:line function "format"
<5> block/blk-settings.c:661 disk_stack_limits "%s: Warning: Device %s is misaligned\n"
<4> kernel/trace/trace.c:8296 trace_create_file "Could not create tracefs '%s' entry\n"
<6> arch/x86/kernel/hpet.c:144 _hpet_print_config "hpet: %s(%d):\n"
<6> init/do_mounts.c:605 prepare_namespace "Waiting for root device %s...\n"
<6> drivers/acpi/osl.c:1410 acpi_no_auto_serialize_setup "ACPI: auto-serialization disabled\n"

, where the meaning is:

  • level:

    log level value: 0-7 for particular severity, -1 as default, ‘c’ as continuous line without an explicit log level

  • flags:

    optional flags: currently only ‘c’ for KERN_CONT

  • filename:line:

    source filename and line number of the related printk() call. Note that there are many wrappers, for example, pr_warn(), pr_warn_once(), dev_warn().

  • function:

    function name where the printk() call is used.

  • format:

    format string

The extra information makes it a bit harder to find differences between various kernels. Especially the line number might change very often. On the other hand, it helps a lot to confirm that it is the same string or find the commit that is responsible for eventual changes.

printk() Is Not a Stable KABI

Several developers are afraid that exporting all these implementation details into the user space will transform particular printk() calls into KABI.

But it is exactly the opposite. printk() calls must _not_ be KABI. And the printk index helps user space tools to deal with this.

Subsystem specific printk wrappers

The printk index is generated using extra metadata that are stored in a dedicated .elf section “.printk_index”. It is achieved using macro wrappers doing __printk_index_emit() together with the real printk() call. The same technique is used also for the metadata used by the dynamic debug feature.

The metadata are stored for a particular message only when it is printed using these special wrappers. It is implemented for the commonly used printk() calls, including, for example, pr_warn(), or pr_once().

Additional changes are necessary for various subsystem specific wrappers that call the original printk() via a common helper function. These needs their own wrappers adding __printk_index_emit().

Only few subsystem specific wrappers have been updated so far, for example, dev_printk(). As a result, the printk formats from some subsystes can be missing in the printk index.

Subsystem specific prefix

The macro pr_fmt() macro allows to define a prefix that is printed before the string generated by the related printk() calls.

Subsystem specific wrappers usually add even more complicated prefixes.

These prefixes can be stored into the printk index metadata by an optional parameter of __printk_index_emit(). The debugfs interface might then show the printk formats including these prefixes. For example, drivers/acpi/osl.c contains:

#define pr_fmt(fmt) "ACPI: OSL: " fmt

static int __init acpi_no_auto_serialize_setup(char *str)
{
      acpi_gbl_auto_serialize_methods = FALSE;
      pr_info("Auto-serialization disabled\n");

      return 1;
}

This results in the following printk index entry:

<6> drivers/acpi/osl.c:1410 acpi_no_auto_serialize_setup "ACPI: auto-serialization disabled\n"

It helps matching messages from the real log with printk index. Then the source file name, line number, and function name can be used to match the string with the source code.